That raises the question of how to determine a count of valid values in a masked array. Can I assume that I can do 'math' on the mask array itself, for example to sum along a given axis and have the masked cells add up?
In my original example, I would expect a sum along the second axis to return [0,0,0,2,0]. Can I rely on this? I would suggest that a .count operator would be very useful in working with masked arrays (count valid and count masked).
Actually masked arrays already have a count method that does what you want: Python 2.2b2 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information.
from pydoc import help import MA x = MA.arange(10) help(x.count) Help on method count in module MA.MA:
count(self, axis=None) method of MA.MA.MaskedArray instance Count of the non-masked elements in a, or along a certain axis.
x.count() 10